A revolutionary is a person who either participates in, or advocates a revolution. Also, when used as an adjective, the term revolutionary refers to something that has a major, sudden impact on society or on some aspect of human endeavor.
The term—both as a noun and adjective—is usually applied to the field of politics, and is occasionally used in the context of science, invention or art. In politics, a revolutionary is someone who supports abrupt, rapid, and drastic change, usually replacing the status quo, while a reformist is someone who supports more gradual and incremental change, often working within the system. In that sense, revolutionaries may be considered radical, while reformists are moderate by comparison. Moments which seem revolutionary on the surface may end up reinforcing established institutions. Likewise, evidently small changes may lead to revolutionary consequences in the long term. Thus the clarity of the distinction between revolution and reform is more conceptual than empirical.
A conservative is someone who generally opposes such changes. A reactionary is someone who wants things to go back to the way they were before the change has happened. Though a reactionary may advocate a popular revolution as a means re establish older institutions (such as Hitler's failed Beer Hall Putsch), the term "revolutionary" is, in practice, rarely applied to figures on the political right. A revolution is also not the same as a coup d'etat: while a coup usually involves a small group of conspirators violently seizing control of government, a revolution implies mass participation and popular legitimacy. Again, the distinction is often clearer conceptually than empirically.
According to sociologist James Chowning Davies, political revolutionaries may be classified in two ways:
"The revolutionary is a doomed man. He has no private interests, no affairs, sentiments, ties, property nor even a name of his own. His entire being is devoured by one purpose, one thought, one passion - the revolution. Heart and soul, not merely by word but by deed, he has severed every link with the social order and with the entire civilized world; with the laws, good manners, conventions, and morality of that world. He is its merciless enemy and continues to inhabit it with only one purpose - to destroy it."
According to the Marxist Internet Archive, a "amplif[ies] the differences and conflicts caused by technological advances in society. Revolutionaries provoke differences and violently ram together contradictions within a society, overthrowing the government through the rising to power of the class they represent. After destructing the old order, revolutionaries help build a new government that adheres to the emerging social relationships that have been made possible by the advanced productive forces."
Revolutionaries may be terrorists, but there is no necessary overlap. Russian revolutionary Vladimir Lenin for "destroying contact between the revolutionaries and the masses of the revolutionary classes of the population, and spreading both among the revolutionaries themselves and the population in general utterly distorted ideas of the aims and methods of struggle against the autocracy." In his controversial essay "Our Morals: The Ethics of Revolution", the Marxist political theorist Norman Geras posited that terrorism should be rejected, but that violence could be justified in the case of extreme social injustice.